The invention concerns a membrane pump with an operating membrane delimiting a conveying space with a supplemental membrane arranged on the side of the operating membrane facing away from the conveying space, with a membrane interspace provided between the operating membrane and the supplemental membrane as well as with a pump drive for oscillating movement of the operating and supplemental membranes in the same direction, whereby the membrane interspace is associated with at least one suction channel for relieving the pressure of the membrane interspace.
In configuring the membrane of a membrane pump, one endeavors to reach an optimum between rigidity and elasticity. While a high elasticity of the membrane is necessary to keep membrane tensions as low as possible, in contrast, at the same time a high rigidity is to be sought so that the membrane does not buckle under the differential pressure load between the membrane upper side and underside, and thus diminishes the drawing space volume and in the opposite case enlarges the dead space volume.
The diminution of the drawing space volume in connection with membrane vacuum pumps takes place especially in the deeper vacuum region. In this area, great pressure differences between membrane lower and upper side arise. While on the membrane lower side, as a rule atmospheric pressure acts upon the membrane underside, the respective evacuation pressure acts on the upper side of the membrane, whereby the maximal pressure differential results from atmospheric pressure minus the ultimate pressure on the membrane.
With the usual membranes of traditional membrane pumps, especially if these membrane pumps operate in the range of the ultimate pressure and large pressure differentials act upon the membranes, it can be stated that the lateral elastic zone of the flexible membrane is buckled by the atmospheric pressure in the direction toward the conveying space. This xe2x80x9cbucklingxe2x80x9d of the membrane leads to the drawing space volume being decisively diminished, which has negative effects on the suction capacity of the membrane pump.
This change in shape is especially marked with two and multiple stage membrane pumps with low ultimate pressures. With these pumps, the lower vacuum stage is most strongly affected since here the greatest pressure differentials arise.
In order to attain an optimum between the desired elasticity and the necessary rigidity of the membrane, in the past, one again and again found more or less good compromise solutions, whereby frequently a good suction capacity could be reached only by allowing for higher membrane tensions.
From DE 40 26 670 A1, a membrane pump is already known, the intake side of which is connected through a connecting line with the crank space of this membrane pump. In order to be able at least to diminish or even to eliminate the pressure differentials on both sides of the operating membrane and not to expose the operating membrane to additional differential pressure-conditioned stresses, the crank space of this previously known membrane pump stands in connection with its suction side.
The membrane pump previously known from DE 40 26 670 A1 has, however, not been able to succeed in practice because the transmission of the drive forces to the crankshaft situated in the crank space and the connection of this crank space with the suction side of the pump presupposes an additional shaft sealing. Such a shaft sealing is nonetheless associated with further friction losses, higher wear and tear and additional performance requirements. A vacuum in the crank space can in addition lead to an outgassing of the bearing grease in the connecting rod bearing, so that the ball bearing possibly runs dry. Since the bearing lubricant in the crank space can extend into the conveying flow through the connecting line, there exists the danger that the conveying medium will become contaminated.
A multiple stage pump apparatus with a turbo molecular pump is already known from DE 43 20 963 C2 which is connected in series after a two stage rotary pump constructed as a hybrid pump in the path of flow. This hybrid pump has a reciprocating piston pump on the medium entry side after which a membrane pump is connected in series for discharging the conveying medium. The cylinder space of the pistons is closed off from the crank space by means of a sealing membrane. Thereby the interspace provided between the piston on the one hand and the sealing membrane on the other are connected with a drain which opens in the conveying flow direction in front of a suction valve of the cylinder pump.
Since this previously known reciprocating piston pump has a piston, the problems arising with an elastic membrane in connection with pressure differential stresses do not appear with this previously known pump. Rather, with this previously known reciprocating piston pump, the interspace between the piston or its associated gasket on the one hand and the sealing membrane on the other (namely when starting this previously known pump apparatus) can be immediately evacuated to the extent that an unwished overflow from the cylinder space of the reciprocating piston pump into the interspace is absent or is largely avoided, and the entire pump apparatus is therefore ready for operation more rapidly during start up.
From DE 43 28 559 C2, a membrane pump of the type mentioned at the beginning is already known which has an operating membrane, a supplemental membrane as well as a membrane interspace provided between the operating membrane and the supplemental membrane. A drain channel opens into this membrane interspace with the aid of which it is possible to bring the membrane interspace to a lower pressure before the drain channel is closed again.
From FR-A-1 292 254, a membrane-compressor is known, that has a working membrane and a supplemental membrane, which define a membrane interspace there between. The known membrane-compressor includes a pressurized inlet channel that is connected to the membrane-interspace. With the help of the inlet channel, a pressure is created in the membrane interspace that supports the working membrane and which lies between atmospheric pressure and the discharge pressure. In order to control the membrane interspace desired pressure and to be able to reduce the standing pressure on the pressurized side of the compressor, a nozzle is located in the inlet channel. The idea of a pressure discharge is not desired in the compressor known from FR-A-1 292 254.
There therefore exists the object of creating a membrane pump of the type mentioned at the beginning that is manufacturable with little expense and which is distinguished, even with a high elasticity of the operating membrane, by a high suction volume, and in connection with which undesired impurities of the conveying medium are avoided as far as possible.
The object in accordance with the invention is accomplished with a membrane pump of the type mentioned at the beginning, especially with the characteristics according to the invention.
With the membrane pump of the invention, the membrane interspace is pneumatically joined at least through one drain channel with the suction side of the membrane pump. Consequently, the membrane interspace is continuously evacuated such that, on the upper side of the operating membrane and on the underside of the operating membrane, the same pressures constantly prevail during the suction phase. Since in this phase consequently no pressure differential is operating between the membrane upper side and underside of the operating membrane, the operating membrane cannot buckle in the direction of the conveying space, and an undesired diminution of the drawing space is avoided. Through the larger drawing space volume, the suction capacity in the intake phase is increased. This has an especially positive action in pressure ranges or suction capacity ranges which lie in the vicinity of the end pressure. The pressure differentials only act upon the supplemental membrane where they can have no negative influence upon the suction capacity of the membrane pump.
Since no differential pressure acts upon the operating membrane of the membrane pump of the invention, this operating membrane can be configured highly elastically without having to fear the mentioned xe2x80x9cbucklingxe2x80x9d of this membrane. Through the more elastic layout of the operating membrane, membrane tensions decrease significantly which once again brings a clear increase in membrane life. Moreover, the shear stress arising in connection with the churning work of operating membrane can be reduced, the effectiveness of the pump can be improved, and a delay in discharge caused by buckling of the membrane is avoided.
With the aid of a more elastic operating membrane, the membrane stroke of the membrane pump of the invention can also be increased. Since no atmospheric pressure is acting on the membrane under side of the operating membrane and the operating membrane therefore no longer strikes noisily on the conveying space in the pump head, noise development in connection with the membrane pump of the invention is considerably reduced, which assumes significance especially in such pumps that are to be used as suction pumps in medical technology.
Since with the membrane pump of the invention, only the membrane interspace provided between operating membrane and supplemental membrane, and not the crank space as well, is joined with the suction side of the pump, and since with the membrane pump of the invention the crank space can also continue, for example, to remain under atmospheric pressure, a special shaft sealing in the region of the crankshaft is not necessary. In addition, a penetration of bearing grease into the conveying stream is not to be expected, and undesired contamination of the conveying medium is avoided with certainty.
An especially simple embodiment in accordance with the invention provides that the membrane interspace is pneumatically joined through at least one suction channel parallel to the conveying space with the pump inlet. With this embodiment, the pump on the one hand sucks through the pump inlet and on the other hand, through the suction channel, out of the membrane interspace.
A refinement in accordance with the invention in contrast provides that the pump inlet is pneumatically joined through the membrane interspace and the suction channel with the conveying space. With this embodiment in accordance with the invention, the intake path runs into the pump interior from the pump inlet through the membrane interspace, the at least one suction channel and the inlet valve into the conveying space.
Here a further embodiment in accordance with the invention of independent significance worthy of protection is provided in that, in the membrane interspace, at least one intake filter and/or noise damping element is provided. Such a membrane pump in connection with which the intake filter and/or noise damping element is arranged in the membrane interspace can be configured especially compactly.
In order additionally to counteract an undesired fluttering of the membranes and a development of noise, it is advantageous if the intake filter and/or noise damping element is manufactured of an elastic material and is acted upon by the operating membrane on the one hand as well as on the other by the supplemental membrane.
Here an especially advantageous embodiment in accordance with the invention provides that the intake filter and/or noise damping element basically fills up the membrane interspace.
The intake filter and/or noise damping element provided in the membrane interspace is associated with a particularly low manufacturing expenditure if it is configured as an open-cell foam element arranged between the operating membrane and the supplemental membrane.
In order to counteract a buckling of the elastic operating membrane in the ejection phase if the pressure on the membrane upper side continually rises in the direction of atmospheric pressure, a preferred embodiment in accordance with the invention provides that the operating membrane is allocated an inherently stable membrane bracing which is held on a connecting rod of the pump drive, and the operating membrane is braced form fitted on the membrane reverse side, at least in a central region.
With two stage pumps, the delivery pressure of the first stage lies significantly below atmospheric pressure, that is, in the discharge phase, the pressure on the membrane upper side of the operating membrane only rises slightly. For this reason, it is especially advantageous if the membrane pump of the invention forms the first stage of a multiple stage, especially a two stage pump or pump facility.
According to a further embodiment of the invention of independent significance worthy of protection, it is provided that the operating membrane and the supplemental membrane are joined in one piece with each other into a double membrane. Here it is appropriate if the operating membrane and the supplemental membrane are joined with each other in one piece through a central spacer, and if this spacer has on its side facing away from the conveying space an undercut fastening opening for inserting a form fitted fastening element connected with a connecting rod of the pump drive.
It is especially advantageous if the operating membrane is configured as a shaped membrane with the upper side of the conveying space sided membrane being form-fitted to the contour of the conveying space in the upper dead center of the pump specified by the pump head.